Mr Gao, who has described being tortured in the past, disappeared halfway through January.
He is believed to have been taken to one of China's notorious "black" jails: a network of secret locations where activitists are abused.
Amnesty International, Human Rights Watch and Human Rights in China issued a joint statement calling on the Chinese government to clarify Mr Gao's situation.
"We are intensely fearful for Gao Zhisheng's safety at this time, given the security authorities' long history of abusing him and his family," said Sophie Richardson, Asia advocacy director at Human Rights Watch. "He has given detailed accounts of how he was tortured in police custody in the past and he may well be suffering more of the same right now."
Mr Gao was the bookmakers' favourite for this year's Nobel Peace prize, although he did not eventually win the award.
A former coal miner and soldier, he became a member of the Communist Party in Xinjiang and took a self-taught law course in 1995. In 2001 he was voted by a government-run magazine as one of China's "top ten lawyers" after he won a string of pro-bono cases on behalf of poor petitioners.
However, he became a target of the authorities after defended a member of Falun Gong who had been sent without trial to a labour camp. In 2005 he renounced his membership of the Communist Party and wrote an open letter to the president, Hu Jintao, and prime minister, Wen Jiabao, urging them to end torture in China.
He was immediately put under 24-hour police surveillance, his law firm was closed and his license revoked. In 2006, he narrowly escaped an assassination attempt, according to Amnesty, but was then arrested and given a three-year suspended sentence for subversion.
"China should immediately release him," said Roseann Rife, Asia-Pacific Deputy Director at Amnesty International.